The invention is directed to a method for manufacturing a cost-reduced, durable three-way catalyst useful to oxidize hydrocarbons, carbon monoxide and reduce nitrogen oxides in exhaust gas generated by a gasoline internal combustion engine operated near the stoichiometric A/F ratio. More particularly, the catalyst comprises a mixed metal oxide of aluminum and rare earths impregnated with precious metal of platinum and/or palladium overlaid with rhodium.
Catalysts are employed in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into nonpolluting gases including carbon dioxide, moisture (H2O), and nitrogen. When the gasoline powered engine is operated in a stoichiometric or slightly rich air/fuel ratio, i.e., between about 14.7 and 14.4, catalysts containing precious metals like platinum, palladium and rhodium are able to efficiently convert all three gases simultaneously. Hence, such catalysts are often called xe2x80x9cthree-wayxe2x80x9d catalysts. Typically such catalysts use a relatively high loading of precious metal to achieve the high conversion efficiency required to meet stringent emission standards of many countries. This makes the catalyst expensive. In countries where the emission standards are less stringent, a durable catalyst which would meet these less stringent standards and also be less expensive catalyst would be desirable.
We have now found a method for making a durable three-way catalyst which has a significantly lower loading of precious metal than conventional catalysts making it less expensive, but which still obtains excellent exhaust gas conversion efficiency under close to stoichiometric conditions. It includes an aluminum-rare earth (mixed metal) oxide carrying precious metal in a particular arrangement, where a relatively high loading of rare earths is combined with a relatively low loading of precious metal. This and other aspects of the invention will be discussed in detail below.
The invention is a method for manufacturing a durable three-way catalyst useful for treating gasoline engine exhaust gases containing hydrocarbons, carbon monoxide, and nitrogen oxides (NOx) . The catalyst has a relatively high loading of rare earths and low precious metals which reduces its cost.
The invention method comprises first coating a substrate with oxide particles comprising (1) a mixed metal oxide and, optionally, (2) 0 to 8 wt. % oxide stabilizers based on the weight of the mixed metal oxide, the metals of the mixed metal oxide consisting essentially of aluminum and rare earth metals. The oxide is coated on the substrate in an amount of 10-30 wt. %, based on the weight of the substrate. In this oxide coating, the rare earth metals comprise cerium (Ce) and lanthanum (La) and, calculating La as La2O3, Ce as CeO2 and Al as Al2O3, the La is 40-70 wt. % of La+Ce and the rare earth metals are 20-60 wt. % of the weight of the Al+rare earth metals. Then according to the method, precious metal is deposited on the oxide coating, wherein first at least one of platinum and palladium are deposited and then an overlayer of rhodium is deposited, in an amount of about 5-35 g/ft3 based on the volume of the substrate.
The catalyst of the present invention thus contains a significant excess of rare earth oxide as compared to precious metal. It is critical to the present invention that the aluminum and rare earths be incorporated in a mixed metal oxide, i.e., the metal atoms are attached to the same or different oxygen atoms in the lattice of the oxide. One preferred way to form the mixed metal oxide is by co-precipitation from a solution as will be discussed herein in detail. According to another aspect of the invention, it is the catalyst made by the process disclosed above and yet another aspect is the method of treating exhaust gases generated by a stoichiometric gasoline engine with the catalyst by contacting the gas with the catalyst.